Meshlet cone cull

  一块Meshlet可以被一个圆锥简易表达，圆锥的表示为：顶点(Apex)、轴(Axis)和顶角（1/2锥角）。

  圆锥的的范围包含了Meshlet所有法线方向，因此如果一个view向量和圆锥范围内所有向量的点积都是负数，就说明整个Meshlet都是背对着相机，可以整体剔除，体现在下图中，就是如果相机在蓝色区域内，就可以剔除这个Meshlet：

image.png

圆锥生成

引用自ComputeCullData方法

1. 计算Axis

  对所有normal生成最小包围球，球心的单位化向量既是圆锥的轴(Axis):

// Calculate the normal cone
// 1. Normalized center point of minimum bounding sphere of unit normals == conic axis
XMVECTOR normalBounds = MinimumBoundingSphere(normals, m.PrimCount);

// 2. Calculate dot product of all normals to conic axis, selecting minimum
XMVECTOR axis = XMVectorSetW(XMVector3Normalize(normalBounds), 0);

2. 去掉退化圆锥

  如果圆锥的顶角是钝角，则没有剔除意义，所以对Axis和Meshlet每个面的法线做点积，如果有小于0的，则代表是退化圆锥：

XMVECTOR minDot = g_XMOne;
for (uint32_t i = 0; i < m.PrimCount; ++i)
{
    XMVECTOR dot = XMVector3Dot(axis, XMLoadFloat3(&normals[i]));
    minDot = XMVectorMin(minDot, dot);
}

if (XMVector4Less(minDot, XMVectorReplicate(0.1f)))
{
    // Degenerate cone
    c.NormalCone[0] = 127;
    c.NormalCone[1] = 127;
    c.NormalCone[2] = 127;
    c.NormalCone[3] = 255;
    continue;
}

  在后续的Amplification Shader中，会拿取到当前数据，并判断假如w分享是0xff，则判断为退化圆椎：

bool IsConeDegenerate(CullData c)
{
    //这里NormalCone4个分量被打包成uint，所以要>>24
    return (c.NormalCone >> 24) == 0xff;
}

3. 偏移顶点

  上图事例中，圆锥顶点都在Meshlet网格的后面，但实际上可能在网格的前面，这时候要将顶点按轴移动到所有Mesh的负半空间：

  首先计算Meshlet的最小包围球（上面是normal，现在是mesh的），默认顶点是这个最小包围球的球心。
  对于每个面，计算原顶点按轴移动，移动到负半轴的距离t，找到最大的t作为顶点偏移值。

  如上图，深蓝色标记着3个面，红色点沿轴移动后，分别在三个粉色点达到平面，需要计算的是红色点和粉色点的距离t，并找到最大值后移动。

  我们有normal、axis是单位向量，找三角形任意一点连接Center，将线段投影到normal，得到center到表面的距离：

上图标识的角度的cos是axis和normal的点积，通过点积和center到面距离，可以算出t

// Find the point on center-t*axis ray that lies in negative half-space of all triangles
float maxt = 0;

for (uint32_t i = 0; i < m.PrimCount; ++i)
{
    auto primitive = primitiveIndices[m.PrimOffset + i];

    uint32_t indices[3]
    {
        primitive.indices.i0,
        primitive.indices.i1,
        primitive.indices.i2,
    };

    XMVECTOR triangle[3]
    {
        XMLoadFloat3(&vertices[indices[0]]),
        XMLoadFloat3(&vertices[indices[1]]),
        XMLoadFloat3(&vertices[indices[2]]),
    };

    XMVECTOR c = positionBounds - triangle[0];

    XMVECTOR n = XMLoadFloat3(&normals[i]);
    float dc = XMVectorGetX(XMVector3Dot(c, n));
    float dn = XMVectorGetX(XMVector3Dot(axis, n));

    // dn should be larger than mindp cutoff above
    assert(dn > 0.0f);
    float t = dc / dn;

    maxt = (t > maxt) ? t : maxt;
}

4. 预计算cutoff阈值

  对于圆锥母线和轴的夹角cos值可以预计算，不过别忘了需要计算的是蓝色区域：

  

// cos(a) for normal cone is minDot; we need to add 90 degrees on both sides and invert the cone
// which gives us -cos(a+90) = -(-sin(a)) = sin(a) = sqrt(1 - cos^2(a))
XMVECTOR coneCutoff = XMVectorSqrt(g_XMOne - minDot * minDot);

  在Amplification Shader中直接比较：

//bool IsVisible(CullData c, float4x4 world, float scale, float3 viewPos)

float4 center = mul(float4(c.BoundingSphere.xyz, 1), world);

// Transform axis to world space
float3 axis = normalize(mul(float4(normalCone.xyz, 0), world)).xyz;

// Offset the normal cone axis from the meshlet center-point - make sure to account for world scaling
float3 apex = center.xyz - axis * c.ApexOffset * scale;
float3 view = normalize(viewPos - apex);

// The normal cone w-component stores -cos(angle + 90 deg)
// This is the min dot product along the inverted axis from which all the meshlet's triangles are backface
if (dot(view, -axis) > normalCone.w)
{
    return false;
}